According to the results of one survey, of vehicles running on the highway, the percentage of tires with substandard maintenance, when compared with the drive train, and the substandard maintenance percentage of other vehicle systems, is unexpectedly high. Breaking down that percentage, unsuitable tire air pressure covers more than half, and is the top cause, followed by lack of tire tread, and continuing with abnormal tire abrasion/wear and tear. Moreover, there are many drivers who have experienced a tire blowout while driving on the highway, and also as a result of the fear of a blowout while driving on the highway, the desire to equip cars with a tire air pressure warning device, or an equivalent device as standard equipment is rising, and, if tire air pressure warning devices can become less expensive, there are also requests to additionally equip cars in present use with tire air pressure warning devices.
On vehicles, including freight trucks, sensors that detect the air pressure of each individual tire have been provided. By way of slipping, electric waves, magneto-electric induction methods, systems that detect the tire air pressure of a rotating tire in real-time are known. According to these systems, an attempt to inform the driver of tire air pressure abnormalities, such as resulting from a tire puncture etc., is carried out. The above system, which detects the tire air pressure in real time, directly detects the tire air pressure, and has high reliability as an advantage, but the slipping, electric wave, electro-magnetic induction methods have the drawback such as, in order to transmit the air pressure information, the entire system is large scale, moreover, becomes very expensive, and has a problem that the actual use of the system is rather poor.
Also, along with the electrical devices becoming inexpensive in recent years, 4 wheel ABS devices and TRC (Traction Control) devices have come into use on passenger cars. In the case of providing 4 wheel ABS devices, and the like, a device for detecting the number of tire revolutions, a tire revolution detection device, is individually provided. It is made-up of a control system, consisting of individual brake and brake hydraulic devices, and utilizing the vehicle behavior, applies appropriate control.
With this, as described in Japanese Patent laid open 63-305011, a detection method for detecting an abnormal tire air pressure is proposed, in which the number of tire revolutions is summed in the diagonal direction, and it is judged as to whether or not the value of the difference between the sum of the two diagonal pairs of the number of tire revolutions and the difference between the number of revolutions between each tire is greater than that of a prescribed value. In order to eliminate the need for extra additional hardware added to the vehicle, for the aforementioned 4 wheel ABS equipped vehicles, by means of the already provided tire revolution detection device, it is thought that the detected number of tire revolutions can be used effectively to realize a tire air pressure warning device and the like.
That is to say, when running in a straight line, if a specific tire suffers a puncture and the air pressure in that tire rapidly decreases, as a result of the punctured tire's outer diameter becoming smaller than that of the other normal tires, the number of revolutions of the tire increases (because when traveling the same distance, the tire with the smaller diameter will revolve much more rapidly) as compared to the other normal tires. From this, detection of a tire that suffers a puncture, or loss in air pressure becomes possible.